Background: Reproductive problems (such as infertility, miscarriage, and birth defects) have been linked to environmental toxins in laboratory animals. However, little is known about similar effects in humans. Human reproduction is not only a likely target for environmental toxins, but it may be a more sensitive endpoint than cancer or other chronic disease, which can require many years to become apparent. Thus, reproductive problems may serve as an early signal of toxic exposures. Our goal is to describe the basic biology of human reproduction, to develop improved epidemiologic tools for detecting environmental damage to reproduction, and to identify specific environmental factors that interfere with human reproduction. Summary: Pregnancy Testing. We estimated the maximum screening sensitivity of pregnancy tests when used on the first day of the expected period, taking into account the natural variability of ovulation and implantation. We conducted a community-based prospective cohort study conducted in North Carolina between 1982 and 1986. Two hundred twenty-one healthy women 21 to 42 years of age who were planning to conceive participated. Main outcome measures were day of implantation, defined by the serial assay of first morning urine samples using an extremely sensitive immunoradiometric assay for human chorionic gonadotropin (hCG), relative to the first day of the missed period, defined as the day on which women expected their next menses to begin, based on self-reported usual cycle length. Data were available for 136 clinical pregnancies conceived during the study, 14 (10%) of which had not yet implanted by the first day of the missed period. The highest possible screening sensitivity for an hCG-based pregnancy test therefore is estimated to be 90% on the first day of the missed period. By 1 week after the first day of the missed period, the highest possible screening sensitivity is estimated to be 97%. In this study, using an extremely sensitive assay for hCG, 10% of clinical pregnancies were undetectable on the first day of missed menses. In practice, an even larger percentage of clinical pregnancies may be undetected by current test kits on this day, given their reported assay properties and other practical limitations. Reliability of hCG. To examine the reliability of HCG as a biomarker for early pregnancy loss, five experienced researchers independently assessed data from 153 menstrual cycles, determining whether each cycle represented 'no conception', a 'continuing conception' or a 'conception lost'. Urine samples were analyzed by immunoradiometric assay using a combination of capture antibodies for the intact heterodimer (B109) and for an epitope common to the beta subunit and the beta core fragment (B204). For each cycle, HCG data were presented as graphs of daily assay results. Summary statistics for HCG assays from 46 women who had undergone bilateral tubal ligation represented baseline values. Pairwise agreement among the assessors for any of the three options ranged from 78-89%. At least three experts agreed for 147 cycles (96%), accounting for 28 conception losses and 19 continuing conceptions. The multi-rater kappa was 0.62 for the conception lost category and 0.68 for continuing conceptions, indicating substantial agreement. The main sources of disagreement involved deciding whether there was sufficient information for assessment, interpreting cycle parameters such as cycle length or bleeding event, and interpreting a distinct HCG rise pattern that does not exceed the baseline value obtained from the sterilized women. Onset of symptoms of pregnancy. 221 women attempting pregnancy made daily records of the presence or absence of symptom s of pregnancy during cycles of attempting pregnancy and during the 8 weeks following the last menstrual period (LMP). Among 136 women delivering live infants, 89% had onset of symptoms by the end of the 8th week (median day 36). Women who smoked tobacco or marijuana had later onset of symptoms. Symptoms also occurred later in women who had clinical miscarriages. Among 48 who lost their pregnancies before 6 weeks LMP, 21% reported symptom onset. Nearly 90% of women with successful pregnancies experience symptoms within 8 weeks LMP. Very early losses (before 6 weeks) are unlikely to be confirmed clinically, but they are sometimes recognized as symptomatic by women themselves.